1. Field of the Invention
The present invention relates to permanent magnet brushless electric motors. More specifically, this invention relates to means for operating a permanent magnet brushless electric motor as an AC synchronous motor with full rated starting torque capability and minimum xe2x80x98amps per unit torquexe2x80x99 under substantially all loading conditions.
2. Related Art
Variable speed permanent magnet (PM) brushless DC (BLDC) motors are commonplace today. The basic prior art BLDC motors incorporate a stator having a 3-phase armature winding and a permanent magnet rotor mounted coaxially within the stator structure. The motor also incorporates some form of rotor shaft or rotor magnetic field position sensor, such as Hall Effect sensors, back EMF sensors, synchronous resolvers, etc.
The sensors act to control the off-on-off timing of electronic power switches controlling the flow of electric current in the 3-phase AC windings of the stator armature to cause a motoring action.
The basic prior art BLDC motors operate on the principle of selectively energizing two of the three phases of the armature winding. Every 60 electrical degrees of rotation, the rotor position or magnetic field position sensor signals the control electronics to de-energize one of the two energized phases and to energize the formerly de-energized phase.
A typical switching sequence is described in the following, with xe2x80x98Axe2x80x99, xe2x80x98Bxe2x80x99, and xe2x80x98C, representing the three phase windings, and xe2x80x98+xe2x80x99, xe2x80x98xe2x88x92xe2x80x99 representing the electrical polarity of the flow of electrical current:
(A+ to Bxe2x88x92), (A+ to Cxe2x88x92), (B+ to Cxe2x88x92), (B+ to Axe2x88x92), (C+ to Axe2x88x92), (C+ to Bxe2x88x92), and repeat (A+ to Bxe2x88x92), - - -
The armature winding of a 2-pole BLDC motor experiences six distinct switching steps for each 360 mechanical degrees of rotation, while the winding of a 4-pole BLDC motor experiences 12 distinct switching steps. As a consequence, the typical present day BLDC motor has inherent torque pulsations.
Further, any error in the timing of the off-on-off action of the electronic power switches relative to the rotor position adds to the torque pulsations of the typical BLDC motor.
Additionally, the forced timing of the switching of the electric current in the armature windings forces the rotor magnetic field to operate at a substantially fixed xe2x80x98torque anglexe2x80x99 relative to the armature magnetic field, regardless of load. This fixed torque angle is the optimum torque angle for only one magnitude of motor torque. At other magnitudes of torque, the motor performs at less than the optimum xe2x80x98torque-per-ampxe2x80x99.
Further, any errors in the placement or position of the sensors relative to the armature windings decreases the torque-per-amp performance of the motor.
Conventional prior art BLDC motors are further limited to an inherent maximum motoring rotational speed determined by the magnitude of the DC voltage applied to the control system. At rotational speeds above the inherent maximum motoring speed, the conventional BLDC motor becomes a generator. Special, and costly, xe2x80x9cfield weakeningxe2x80x9d circuits, conducting large electric currents, are required to overcome the magnetic strength of modern day permanent magnets to enable conventional prior art BLDC motors to operate as a motor at rotational speeds greater than the rotational speed at which the motor would otherwise become a generator.
In engine driven systems, it is desirable to have a single electric motor-generator system to provide both the engine starting function and the auxiliary electric power generation function. Many small and medium aircraft have engine systems that include a 28-volt DC brush-and-commutator motor-generator (starter/generator) to provide the engine starting function and to also provide the aircraft electric power generation function.
The 28-volt DC brush-and-commutator starter/generators used on present day helicopters and general aviation aircraft require frequent and expensive maintenance relating to the brushes and commutator. It would be advantageous to provide a brushless 28-volt DC starter/generator system to reduce the maintenance cost.